A description of the benefits of variable valve actuation is given in U.S. Pat. No. 5,456,224 by Riley. Optimal operation of valves requires suitable variation of lift, duration and phase. Many of the benefits desired can be achieved by suitable variation in phase alone.
Variation of phase has been accomplished in a number of different ways. One of the simplest methods to comprehend is to alter the phase between the crankshaft and the intake camshaft in a dual overhead camshaft layout. This may occur with a drive mechanism that incorporates splines on the camshaft and a driving drum, as shown in SAE paper 901727. During operation the driving drum and camshaft are moved relative to each other while the cam drive undergoes normal operation.
Examples of phase shift using axial, three-dimensional cams, and cam switching are described in U.S. Pat. No. 3,618,574 by Miller, and U.S. Pat. No. 4,970,997 by Inoue et al.
Phase shifting for internal combustion engines where the valves are actuated via pushrods is more limited. Pushrod engines generally use one camshaft only. Cam phasers based on changing the angular relationship between the crankshaft and camshaft, as described in SAE paper 901727, cause identical change in the timing of all valve events, both intake and exhaust.
Phase shifting based on the Clemson camshaft described in U.S. Pat. No. 4,770,060 allows variation of selected lobes on the camshaft, while retaining the normal drive mechanism. Independent phasing of intake and exhaust can be achieved by incorporating cam drive phasing as well.
Another approach is shown in U.S. Pat. No. 2,266,077 by Roan. As part of his variable valve mechanism, Roan interposed a movable roller between the camshaft and the follower under the control of a lever to the side. Movement of the lever resulted in a change of the phase of the valve events relative to the crankshaft. A similar approach is taken by Smith in U.S. Pat. No. 2,851,851.